Referring to FIGS. 6a to 6d, a conventional chip mounting technique of mounting a semiconductor chip onto a board is described, and a conventional chip removing technique of removing a defective semiconductor chip from a board is depicted in FIG. 7.
As shown in FIG. 6a, a die bonding resin 3 which is a thermosetting resin is applied to a die bonding pad 8 arranged on a board 1, which is a preparatory step to place a semiconductor chip 4 onto the board 1. A board electrode 2 is provided on top of the board 1.
Next, as shown in FIG. 6b, the semiconductor chip 4 is placed onto a lump of the die bonding resin 3 in such a way that the die bonding resin 3 spreads uniformly. Then, the die bonding resin 3 hardens by the application of heat, so that the semiconductor chip 4 is firmly fixed to the board 1. In the figure, a bonding wire 5, a chip electrode 6 provided on the semiconductor chip 4, and a bonding capillary 7 are shown.
Next, as shown in FIG. 6c, the chip electrode 6 and the board electrode 2 are connected together electrically by the bonding wire 5 of gold (Au) or aluminum (Al).
Finally, as shown in FIG. 6d, a molding resin 9 is applied to the semiconductor chip 4 from above by means of, for example, potting. When the molding resin 9 hardens, a finished semiconductor device is obtained.
In the above-described wire bonding technique, supersonic wave as well as load is applied to the bonding capillary 7, with one end of the bonding wire 5 and the board electrode 2 in contact. As a result, the board electrode 2 and the bonding wire 5 are interconnected. In the same way, the other end of the bonding wire 5 and the chip electrode 6 are connected together. If the bonding wire 5 is made of Au, its connection strength to each electrode is further improved by the application of heat to the connections. The diameter of the bonding wire 5 is somewhere between 20 to 35 .mu.m. If the position of the bonding capillary 7 can automatically be determined by numerical control when connecting the bonding wire 5 and each electrode, this improves productivity.
The molding resin 9 is a resin having insulating properties (for example, an epoxy resin). The molding resin 9 preferably has a viscosity low enough to prevent the bonding wire 5 from being swept away by the flow of the molding resin 9 during the molding process, and thus possible short circuiting between the bonding wire 5 and an adjacent bonding wire is preventable. The viscosity of the molding resin 9, however, should be high enough to prevent the resin 9 from spreading too widely.
The semiconductor industry is now trying to increase the packaging density of multi-chip modules (MCM) in which many different semiconductor chips are mounted on a single board by employing a wire bonding technique in mounting semiconductor chips on a module board.
As described above, many different chips are mounted on a single board in the case of MCM, so that the final MCM yield can be expressed as the product of yields of individual semiconductor chips. This is the reason that as the number of semiconductor chips increases the MCM yield decreases.
If a defective semiconductor chip is found on a module board, this leads to loss of the entire MCM including all the other good chips. Therefore, to prevent a defective chip from causing a fatal effect on the entire MCM, defective chips must be removed from the board. Then, a good one is mounted where the detective chip was mounted by means of wire bonding.
The problem is that no conventional techniques can effectively remove defective semiconductor chips if they are mounted onto a board by means of wire bonding techniques.
Conventionally, a defective semiconductor chip is removed from a board as follows (FIG. 7). The bonding wire 5 is held by a pair of tweezers 10 or the like at around the middle, and is picked up, and is removed from the board. Next, the adhesive strength of the die bonding resin 3 is weakened and the semiconductor chip 1 is removed from the board 1.
This chip removing technique, however, produces several problems as follows:
(a) When a bonding wire is in tension, it is likely to cut easily, and thus an unwanted wire piece may be left on the board electrode 2. If this wire piece is swept away by the flow of a molding resin supplied at a later molding process, this may cause interwiring short circuiting; and PA1 (b) Since there are difficulties in automatically removing a wire from a board, this leads to poor productivity. PA1 (a) For the case of MCM, many different chips are mounted on a single board so that the MCM yield can be expressed as the product of yields of individual semiconductor chips. This is the reason that as the number of semiconductor chips increases the MCM yield decreases; and PA1 (d) Only a single faulty semiconductor chip can damage the entire MCM. This leads to loss of the entire MCM including a module board and all the other good semiconductor chips.
These problems produces further problems as follows: